The long term objective of the laboratory is to understand the biochemical events underlying the invasion of the P. falciparum merozoites into its host cell, the erythrocyte. We have previously identified and characterized in detail the surface molecules that mediate recognition between the merozoite and the erythrocyte. We are now interested in extending this work to study the mechanism whereby the merozoite penetrates the erythrocyte. This is particularly intriguing phenomenon, because of the rigid nature of the erythrocyte membrane and its lack of phagocytic or endocytotic ability. The parasite must initiate its entry. Central to the invasive process is the rhoptry organelle, located at the apical end of the merozoite. Electron microscopic studies suggest that the contents of the rhoptries are secreted during merozoite invasion. Except for such studies, little is understood about its biochemical role in invasion. We have identified a protein of M.W. II0,000,Pf II0, as a component of the rhoptries by reactivity with a MAb. During merozoite invasion, the PfII0 is secreted into the erythrocyte membrane. Preliminary experiments suggest that PfII0 is associated with protease activity. The specific objective of the present proposal is to perform a systematic analysis of the components of the rhoptries. The organelle will be isolated from merozoites and the protein and enzyme activity analyzed. The interaction of specific rhoptry polypeptides with membrane proteins of the erythrocyte will be investigated, both in vitro, and in the membrane of infected erythrocytes. Synthetic inhibitors of rhoptry proteases will be tested for their effects on merozoite invasion. We anticipate that these experiments will lead to an understanding of the functionally important domains of the rhoptry proteins. Such a site could represent a conserved target for immune attack in individuals infected with malaria, and a corresponding synthetic sequence form the basis of a therapeutic reagent.